A respirator of this type has become known from DE 195 16 536 A1. The prior-art respirator is used for pressure- and volume-controlled forms of respiration, the breathing strokes being triggered either by a timing control or by the patient's spontaneous breathing activity. It is often necessary in respiration therapy and respiration diagnostics to raise the CO.sub.2 level in the blood in order to thereby stimulate the breathing. The enrichment of the CO.sub.2 may be achieved, e.g., by the partial rebreathing of the previously exhaled gas. The pulmonary blood flow can be determined in a noninvasive manner by specific CO.sub.2 rebreathing.
The CO.sub.2 content in the inhaled air can be influenced by an additional volume connected into the breathing gas line in the vicinity of the patient, by which the dead space in the breathing gas line is increased. Such a device for increasing the dead space has been known from DE 25 07 981. The drawback of the prior-art dead space increase is that it must be arranged in the vicinity of the patient connection and cannot be used for routine applications because of its relatively large volume. Since the respiration is usually performed with breathing gas enriched with water vapor up to the saturation, a large amount of condensate is produced in the dead space volume, and this condensate must be disposed of continuously.
In the device known from WO 98/12963, the line section between the Y piece and the patient connection is split into two lines, between which switchover is possible by means of a 3/2-way valve, one of the lines having a larger dead space for rebreathing than the other line. This device also has the drawback that the additional dead space volume is located in the vicinity of the patient connection with the handling and condensation problems associated therewith.